1. Field of the invention
The present invention relates to a photochromic material for use in optical storage media and other areas of application.
2. Description of Prior Art
Substances which display reversible color changes upon exposure to light are collectively known as photochromic materials. Spiropyran compounds constitute one of the most intensively studied types of photochromic material.
Many spiropyran compounds have already been reported in literatures. For example, the colorless spiropyran compound A of the following formula is transformed into the red compound merocyanine B by irradiation with ultraviolet rays of wavelength approximately 340 nm. The compound B reverts to the form A if irradiated with visible light with a wavelength of approximately 580 nm. ##STR2##
Optical storage media can be prepared by utilizing these photochromic materials which change their structures by irradiation. In order to miniaturize devices containing optical storage media, the use of semiconductor lasers is generally desirable. The semiconductor lasers employed in optical storage devices ordinarily emit light in a wavelength region in the neighborhood of 700 nm, and therefore the colored form of photochromic materials used for such purposes should desirably possess absorption sensitivity in this wavelength region.